This invention relates to a multi-fuel engine fuel control and particularly to a closed loop fuel control using an exhaust sensor such as an oxygen sensor which generates an output signal voltage varying with air/fuel ratio through a region around stoichiometry. In the process of generating a feedback control signal, the output signal voltage of the exhaust sensor is compared with a reference or reference window to determine the rich/lean status of the fuel mixture. The feedback signal is used to modify a basic fuel quantity calculated to produce a desired stoichiometric air/fuel ratio for delivery to the engine combustion chambers.
The fuel is a mixture of two fuels such as gasoline and methanol that have different volumetric heat contents and thus different stoichiometric air/fuel ratios. The control may include a fuel composition sensor responsive to a physical component of the fuel mixture to generate a fuel composition signal indicative of the relative proportions of first and second combustible fuels therein; and the fuel composition signal may be used in the calculation of the fuel quantity to compensate for the varying stoichiometric air/fuel ratio of the fuel mixture.
However, it has been found that an oxygen sensor of the type commonly used in closed loop fuel control systems is affected by the inclusion of methanol in the fuel mixture so that it shifts its output voltage slightly in the rich direction with increasing methanol concentration. The shift is small --on the order of 0.1-0.2 air/fuel ratios--but it tends to run the engine on the lean side of stoichiometry; and a slight shift to the lean side can have a significant affect on NO.sub.x conversion by a reducing catalyst in the engine exhaust.